Height adjustable work chairs are used in home, office, education, and industry. These height adjustable chairs typically include a seat that swivels in relation to the base of the chair. The height adjustment is typically obtained from a telescoping height adjustment mechanism which is usually a gas spring. This gas spring is a telescoping column that includes a gas cylinder which includes a pressurized gas such as nitrogen; a piston extending downwardly from the cylinder which supports the cylinder in an elevated position and includes an end which secures the gas spring to the chair base; and a valve located within the cylinder which allows the movement of gas within the cylinder.
The top of the cylinder is secured within the seat support mechanism which is the mechanism that supports the chair seat and chair back. The seat support mechanism is also known as the chair tilt mechanism, particularly on chairs with adjustable backrests or chairs having a tilting chair seat. Also included in the seat support mechanism is a lever that actuates the valve located in the gas spring. The valve typically includes a pin extending from the top of the cylinder which when depressed by the lever allows gas to move through the valve and lower the cylinder if a force exceeding gas pressure is applied on the spring or raise the cylinder if no force is applied on the spring. The gas spring may additionally include an air or oil damping mechanism which slows the rate of compression and extension for the gas spring.
The gas spring which is responsible for height adjustment in work chairs also allows the seat to swivel in relation to the base. The gas spring cylinder which is secured to the chair seat through the seat support mechanism rotates freely in relation to the piston which supports it. It is this rotation which provides the swivel in most work chairs. Additionally the end of the piston which is attached to the chair base may also include ball bearings or other means to allow the piston to rotate in relation to the base while remaining secured to the base. In this way, the gas spring which is a very inexpensive mechanism to manufacture provides both height adjustment and swivel movement of the chair seat in relation to the base.
The gas spring is supported in a vertical position between the chair base and the chair tilt mechanism by a vertical stand tube. The stand tube includes a top opening sized so that the cylinder of the gas spring can move vertically and rotationally in relation to the stand tube, yet remain supported by the stand tube regardless of the vertical or rotational position of the cylinder.
The stand tube is typically a metal cylinder which includes top and bottom ends. An opening on the top end which is sized to accommodate a gas spring cylinder which moves vertically in relation to the stand tube. In some instances a bushing may be disposed within the opening to minimize friction between the gas spring cylinder and the stand tube. The opening serves to both guide the gas spring cylinder as well as to vertically support the gas spring which extends between the chair seat support mechanism and the chair base.
The bottom end of the stand tube cylinder typically is tapered for insertion into the chair base. The tapered bottom end is typically pressed into a tapered socket at the hub or center of the chair base. The stand tube bottom may include an opening through which the piston of the gas spring can extend so as to be secured to the chair base. Alternatively, the stand tube bottom end can include an appropriate structure for securing the gas spring piston. This structure also may include an opening in the stand tube bottom end.
An alternative to the gas spring is a metal coil type spring which includes an oil damping mechanism. The application of a spring of this type is shown in U.S. Pat. No. 5,078,351. An alternative to the typical stand tube arrangement is shown in U.S. Pat. No. 5,433,409. The inventors of both of these alternative chair components believe that their components address structural deficiencies found in existing chair component assemblies.
These structural deficiencies are most often derived from the use of a gas spring as both the structural support for the chair as well as the mechanism which provides vertical adjustability to the chair. The dual purpose of the gas spring, as well as the manner that the gas spring is supported, and the manner that the gas spring is attached to the chair seat support mechanism and the chair base, may cause the gas spring or the supporting structure for the gas spring to fail.
Gas spring mechanisms are remarkably strong despite the light gauge metal used in their construction. The cylinder portion of the gas spring, which is typically constructed of light gauge metal, is a sealed assembly which includes an internally held compressed gas. The cylinder also comprises the structure which provides the attachment between the height adjustable chair pedestal and the chair seat. Typically, the cylinder of the gas spring is attached to the seat support or chair tilt mechanism within a tapered socket or a hole within the seat support or chair tilt mechanism. Accordingly, all of the weight on the chair seat must be transferred through the light gauge cylinder wall. The forces exerted on the cylinder may include high bending moments, if the weight of the chair user is not sitting on center of the chair seat, in addition to the compressive forces acting downward on the cylinder of the gas spring due to the users weight.
These compressive and bending forces may result in considerable stresses exerted on the light gauge cylinder walls which can result in a catastrophic failure of the cylinder. Additionally, the concentration of stresses in each of the chair components results in shape distortions to occur in these components. These distortions result in play or looseness developing, particularly at the high stress attachment points of the components. Play is known to develop particularly early at the attachment location of the cylinder to the seat support or tilt mechanism. This play, once created, can further accentuate stress concentrations at specific locations on the cylinder wall which also can lead to a catastrophic failure of the cylinder wall.
Similar forces exist between the gas spring cylinder and the stand tube. As has been previously described, the stand tube supports the gas spring by providing a top opening through which the cylinder of the gas spring passes. The gas spring cylinder moves vertically, as well as rotationally in relation to this opening. The stand tube may include a bushing to reduce friction between the cylinder and the stand tube.
Forces exerted on the gas spring cylinder may be transferred to the stand tube at the opening location. As the opening must be made larger than the cylinder to permit both vertical and rotational movement of the cylinder, these forces may be transferred from a small segment of the thin wall of the cylinder to the stand tube along the short support surface provided by the opening. This concentration of forces can cause failure of the cylinder either by shearing the cylinder wall or by causing a small dent in the cylinder wall which could lead to the cylinder buckling at a later time. Forces that are not transferred to the stand tube are transferred first to the piston of the gas spring cylinder and then to the chair base. The forces on the piston can also cause the small diameter rod of the piston to buckle.
Other stand tubes include a honeycomb structure within the interior of the stand tube cylinder to prevent a concentration of forces on the gas spring cylinder at an opening. In this design, forces on the gas spring that are transferred to the stand tube are transferred over a large surface of the honeycomb structure within the stand tube interior. The honeycomb structure flexes in response to such forces assuring a large contact surface for force transfer and minimizing shear forces on the gas spring. The excessive flexure within this design results in a chair having a non-rigid pedestal having a great deal of play. Accordingly, chairs having this construction do not appear sturdy and durable to consumers. Additionally, the flexing of the gas spring allowed by the flexible honeycomb structure may also cause the gas spring to eventually fail. It is also likely that this excessive flexing of gas springs may contribute to gas leakage from the gas spring.
Additionally, the forces resulting from pressurized gas within the cylinder are increased by forces on the chair seat which also are transferred through the gas spring. Gas losses through the cylinder seals are affected by the forces exerted on the gas spring. Should forces be applied to the gas spring which are either too high for the gas spring or are applied at an excessive rate, the gas spring seals can also fail, resulting in the immediate loss of gas pressure and a sudden complete compression of the gas spring. Such a failure could also result from normal use of the chair over a long period of time. Gas losses resulting from a progressive loss of the high pressure gas are evident in all gas springs and are most pronounced in those having the highest initial gas pressures.
Other areas where failures can occur are at the chair base hub socket where the stand tube is attached to the chair base. This area is particularly prone to failure in chairs having a one piece molded base that includes a center hub having a tapered socket for receiving the tapered bottom end of the stand tube, and a plurality (typically 5) of spokes which radiate from the hub. The molded plastic hub socket wall supports the rigid metal stand tube in a vertical position. Forces transmitted through the stand tube are transferred to the hub at this location. The high strength, and highly rigid steel stand tube may first become loosely held within the plastic chair base hub socket due to normal use of the chair. The socket wall also may be prone to breakage due to material fatigue stresses, or as a result of the uneven application of forces on the socket, which result from play within this joint. All metal chair bases are also prone to developing play in this joint, but are understandably less likely to fail.
All of the described modes for height adjustable chair pedestals may result in injuries to the chair user as well as property damage. Obviously, a catastrophic failure of a chair may cause the person on the chair to fall from the chair without any warning.
All of the previously mentioned problems with height adjustable pedestals used in office type chairs have prevented the widespread use of similar height adjustable pedestals in tables. Off center forces exerted on a table top close to the perimeter of the table top are a considerable distance from the center pedestal supporting the table top. Such forces obviously result in high bending moments on a small diameter gas spring cylinder which is incorporated into the pedestal for supporting a vertically adjustable table top.
Due to the current limitations of gas spring height adjustment in table constructions, height adjustable table manufacturers have typically used either hydraulics or electric motors to provide vertical adjustability. Such mechanisms, which have been in use for decades, are typically very durable and do not have failure modes consistent with gas spring height adjustment. These mechanisms, however, are not in widespread use because of their considerable expense. Tables that incorporate these devices are, or course, very expensive which limits the use of height adjustable tables in residential, office, educational, and industrial environments.
For the foregoing reasons, there is a need for a height adjustable pedestal for use with chairs and tables commonly used in homes, offices, schools, and industry which provide the benefits of height adjustment at an economical price and are not subject to dangerous catastrophic failures.
In particular, there is a need for a height adjustable pedestal that allows forces to be transferred from the chair seat or table top to the floor which will not cause excessive concentrations or stresses within the elements of the height adjustable pedestal which can lead to catastrophic failure of these elements.
There is a need for a height adjustable pedestal for chairs and tables that are able to be assembled from currently available low cost components, such as gas springs, yet overcome the deficiencies of these components. There is a further need that existing low cost assembly practices can be used in the chairs and tables using these height adjustable pedestals.
There is also a need for a height adjustable pedestal that is constructed to minimize wear in the components of the pedestal assembly. By minimizing wear, such chairs and tables would not develop excessive play within the structural joints connecting components of the pedestal assembly.
There is a need for a height adjustable pedestal that is low cost, low in weight, high in strength and durability, and high in aesthetics which will provide greater widespread use of height adjustment in both tables and chairs.